HR: 1340h
AN: V53B-1562    [Abstracts]
TI: The Basalt of Yellowjacket Butte, Another Large and Interesting Lava Flow at Medicine Lake Volcano, N. California, USA
AU: * Donnelly-Nolan, J M
EM: jdnolan@usgs.gov
AF: USGS, 345 Middlefield, Menlo Park, CA 94025 United States
AU: Champion, D E
EM:
AF: USGS, 345 Middlefield, Menlo Park, CA 94025 United States
AU: Ramsey, D W
EM:
AF: USGS, 345 Middlefield, Menlo Park, CA 94025 United States
AU: Lanphere, M A
EM:
AF: USGS, 345 Middlefield, Menlo Park, CA 94025 United States
AB: The late Pleistocene basalt of Yellowjacket Butte (BYB) covers ~300 km² on the SE flank of Medicine Lake volcano (MLV) in the N CA Cascade Range. Combined field mapping, geochemistry, paleomagnetism, and Ar dating provide constraints on growth of this extensive basalt flow in space and time. Multiple vents on the upper S flank fed lobes via lava tubes to >30 km. Estimated volume of this compositionally-zoned plag-ol basalt is 4-5 km³, most in the earliest lobes. The E lobe is largest, covering >one 7.5' quadrangle. It contains 50.0-51.4% SiO₂, 7.4-8.3% MgO, and ranges in Mg# from 62-66. The smaller SW lobe overlaps the E lobe in composition, but is somewhat less mafic (50.7-51.7% SiO₂, 7.0-7.6 MgO, Mg# 61-64). Both display late, low-volume, high-Sr facies (400- 465 vs. 350-400 ppm for the main lobes). Overlying the high-Sr E and SW lavas are the Hole-in-Rock (HR) lobe and the youngest lava, the NW lobe. The HR lobe typically has higher and more variable SiO₂ (50.4-53.1%), lower MgO (5.2-6.3%) and lower Mg# (50-55), whereas the NW lobe has the lowest SiO_{2} (49.3-50.7%) despite its mid-range MgO (6.8-7.8%) and Mg# (56-61). Sr concentration is similar in both youngest lobes (370-430 ppm), which are partly buried by late Holocene lava, but have ~subequal volumes, larger than either of the late high-Sr facies of E and SW lobes. BYB is distinct from other large MLV compositionally-zoned basalts in having little compositional variation in SiO2, despite considerable (although complex) variability in TiO2, FeO* (7.5-10.2%), K2 O (0.2-0.9%), and other elements. Paleomagnetic sampling at 19 sites indicates a correlation between stratigraphic position, chemical composition, and magnetic inclination (I) value. A >10° shift to shallower I values was found, while declination (D) values were essentially constant as the eruption progressed. The farthest-traveled early E lavas have the highest I (78-80°), whereas the SW and near E lavas have I=75-77°. TiO2 contents for these earliest lavas range from 0.85-1.02%. Three sites in late high-Sr lavas yielded I of 72-74° and higher TiO2 (0.96-1.16%). Three sites in the HR lobe and five in the last-erupted NW lobe yielded indistinguishable I averaging 67 °. HR lavas have the highest TiO2 (1.24-1.45%), while the NW lavas have 1.06-1.20%. The large range in I values combined with unchanging D values from earliest to latest lavas indicates that eruption duration may have been as much as a few hundred years. BYB is unusual in displaying significant variability in remanent magnetic direction, in contrast to limited variation in magnetic direction in other large-volume basalts we have studied at MLV, Newberry Volcano, and elsewhere. The ⁴⁰Ar/³⁹Ar dating of one BYB sample yielded a 1-σ plateau age of 86±14 ka, an isochron age of 76±21 ka, and an inverse isochron age of 73±31 ka. These ages overlap the ~80 ka age of the Norwegian Sea paleomagnetic event and its inferred on-land equivalent, defined as the Badlands Excursion at Newberry Volcano OR (Champion et al., 2004 AGU abstract). It is possible that BYB took no more time (decades) to erupt than most basalts of similar volume in similar tectonic environments. Thus, this basalt may record faster geomagnetic secular variation during an early or late part of the Badlands Excursion. This leads us to speculate that eruptive events may have been roughly synchronous at MLV and Newberry Volcano, two volcanic systems located in similar extensional environments at the transition between the Cascades and the Basin and Range.}
DE: 8178 Tectonics and magmatism
DE: 8185 Volcanic arcs
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8425 Effusive volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005